Multiple well filtration plates such as ninety-six well plates containing the standard eight by twelve array are commonly used to filter fluids using either vacuum or centrifugal force for a variety of applications. Ultrafiltration membranes typically require more force than generated by vacuum to filter a fluid and have to be run using centrifugation or pressure. Centrifuges adapted with rotors and carriers for the ninety-six well microtiter plates are very common laboratory equipment and therefore frequently used to generate the forces required to drive filtration. Viscous, high protein containing serum or plasma is one of the fluids requiring ultrafiltration. In one application, protein free filtrate from serum is required for drug analysis while in another protein-bound drug is conveniently separated from the free drug. This is a critically important determination of how much drug is free and, therefore, available to enter the targeted tissues or organs. The ultrafiltration membrane is a very effective means to remove the high molecular weight material from the small molecules (e.g. drugs) typically being analyzed. These types of experiments are performed in pharmaceutical and biotechnology companies as part of the secondary characterization or ADME/tox. Serum and plasma represents one of the more difficult fluids to ultrafilter and commonly utilizes the forces generated by a centrifuge to produce the “protein-free” filtrate required for analysis. When serum is separated using both centrifugation and ultrafiltration a characteristic filtrate volume pattern is observed. We have called this the “smile effect” and is shown in FIG. 1.
Many of the centrifuge rotors that are used for multi-well plates are swinging buckets. When filtering fluids, multiple well centrifugal filtration devices are spun with all membranes of the well plates in the same plane. All multi-well plates are centrifuged with the center of the major plane of the filter perpendicular to a swing radius vector of the centrifuge. Filtrations of some, especially viscous samples, (e.g. serum or plasma) can take a long time to collect even a minimum filtrate volume. On the other hand, the volume of the filtrate is not homogeneously distributed through the plate. Generally, outer wells of the plate have more filtrate volume and the inner rows of the plate have the least amount of filtrate, thereby causing an uneven filtrate distribution or “smile effect”, as shown in FIG. 1. It is known that the filtration of proteins such as serum result in reduced filtration rates known as concentration polarization or fouling which is caused by forcing these large biomolecules on the active membrane surface. This effect also occurs in swinging bucket rotors since there is no control in the concentration polarization of the membrane.
Therefore, a need exists for an apparatus and method to form a more uniform distribution of filtrate volume in all the wells in addition to controlling the polarization and creating a faster filtration rate.